A conventional device for automatically adjusting the tension of a fan belt (an automatic tensioner) will be described with reference to FIGS. 4 and 5. In the drawings, numeral 101 indicates a crank pulley, which is firmly attached to the tip end of the crankshaft of an engine (not shown). Numeral 102 indicates a fan pulley, which is rotatably attached to the tip end of an shaft firmly attached to a bracket of an engine front cover (not shown), and to an end surface of which a fan 103 is attached. Numeral 104 indicates a tension pulley, which is rotatably attached to the tip end of a lever 105 fitted onto a shaft section 106 firmly attached to the bracket of the engine front cover (not shown) and adapted to make a swing motion. Further, a coil spring 107 is wound around the shaft section 106 of the lever 105, and, by the reaction force of this coil spring, the tension pulley 104 is pushed against a belt 108, thereby imparting tension to the belt 108.
The operation of this device will now be described. During normal operation of the engine, the belt 108 is driven by the crank pulley 101, and, due to the resistance of the fan 103, a belt portion 108a receives a tensile force and tightens, whereas a belt portion 108b slackens. However, the tension pulley 104 moves in the direction of the arrow F due to the action of the coil spring 107 attached to the shaft section 106, and pushes the belt portion 108b to impart tension thereto, so that no slackening occurs. If the engine suddenly accelerates, the acceleration time is long, so that the tension pulley 104 follows the belt portion 108b to impart tension thereto, thus preventing slackening.
At the time of sudden deceleration of the engine, the deceleration time is short, so that the tension pulley 104 cannot follow the belt portion 108b. Further, while the crank pulley 101 on the driving side rapidly decelerates, the fan pulley 102 on the driven side cannot decelerate rapidly due to the inertia of the fan 103, with the result that the fan pulley 102 is turned to the driving side and the crank pulley 101 to the driven side. As a result, the belt portion 108a becomes the slack side, and the belt portion 108b the tight side. Due to the tight belt portion 108b, the tension pulley 104 moves in the reverse direction R, thereby causing the belt portion 108a to further slacken. As a result, the crank pulley 101 and the belt 108 become liable to slip and cause so-called "creak."
The present invention has been made with a view to solving the above problem. It is accordingly an object of the present invention to provide an automatic tension adjuster for a fan belt which adjusts the tension of a fan belt even at the time of sudden decrease in speed of the engine so as to prevent slip, so-called "creak" or the like from occurring between the fan belt and a crank pulley.